Most modern HVAC systems are employed at sites at which commercial natural gas service is available. As a result, most gas furnaces are designed to use natural gas as their principal fuel. A typical gas furnace employs a controlled solenoid valve to turn the gas on and off as heating needs require; one or more gas orifices to introduce the gas into the furnace in a controlled manner; and one or more heat exchangers to receive the gas, allow air to become entrained and mix with it, contain a combustion of the gas-air mixture and allow heat resulting from the combustion to be transferred to the environment surrounding the heat exchanger.
Though most gas furnaces are designed to use natural gas as their principal fuel, many are also capable of using an alternative fuel, such as gaseous propane. (Propane is typically stored as a liquid in a tank, and propane gas is extracted from the tank to fuel propane-fueled appliances such as furnaces.) This makes such furnaces employable at sites where commercial natural gas service is not available and therefore increases their utility. It is understood, however, that the heat exchangers of furnaces designed to use natural gas have a shorter life expectancy when propane is used instead.